1. Field of the Invention
The present invention relates to an optical head for recording and reproducing information on and from an optical record medium such as an opto-magnetic record medium.
2. Description of the Related Art
FIGS. 1 and 2 show a general construction of a known optical head for use in an apparatus for recoding and reproducing information on and from an opto-magnetic record medium, in which a linearly polarized light beam emitted from a semiconductor laser 50 is made incident on a beam splitter 51 as a P-polarization light beam, and a part of this light beam is transmitted through the beam splitter 51 and is converted into a parallel light beam by a collimator lens 52. The parallel light beam emanating from the collimator lens 52 is made incident upon a reflection mirror 53 which is inclined by 45 degrees with respect to a plane of the drawing of FIG. 1, so that the light beam is deflected by 90 degrees and is made incident upon an objective lens 54. Then, the parallel light beam is focused by the objective lens 54 onto a recording surface of an opto-magnetic record medium (not shown) as a fine spot.
In the opto-magnetic record medium information is recorded as a magnetizing direction, and when the linearly polarized light beam is reflected by the record medium, the polarizing direction is rotated in opposite directions depending on the magnetizing direction by the well known Kerr effect. The reflected light (return light) reflected by the opto-magnetic record medium again travels back along the optical axis onto the beam splitter 51 through the object lens 54, reflection mirror 53 and collimator lens 52.
The return light beam impinging upon the beam splitter 51 includes an S-polarization component, because the polarization plane of this light beam has been rotated by the Kerr effect. A substantial part of the S-polarization components is reflected by the beam splitter 51, and a small part of a P-polarization component is reflected by the beam splitter 51.
The return light reflected by the beam splitter 51 is transmitted through a concave lens 55 and is then transmitted through a half wavelength plate 56, so that the polarizing direction of this light beam is rotated by 45 degrees. Then, the light beam emanating from the half wavelength plate 56 is made incident upon a polarization beam splitter 57. The introduced beam is split into the P-polarization component and the S-polarization component by a flat beam splitting surface 57a in the polarization beam splitter 57. The P-polarization component transmitted through the beam splitting surface 57a is made incident upon a first light receiving element 58a of a photodetector 58, and the S-polarized component reflected by the beam splitting surface 57a and further reflected by a reflection surface 57b of the polarizing beam splitter 57 is made incident upon a second light receiving element 58b of the photodetector 58. By suitably processing output signals generated from the first and second light receiving elements 58a and 58b, it is possible to derive an information signal, a focusing error signal and a tracking error signal.
FIG. 2 is a plan view of the photodetector 58 having the light receiving elements 58a and 58b, each of which is divided into three strip-shaped light receiving regions. The principle of detecting the above mentioned signals is well known in the art, so that its explanation is omitted here.
In the conventional optical head, the polarization beam splitter 57 for splitting the return light reflected by the opto-magnetic record medium is arranged in a converging light beam. Therefore, incident angles of light rays impinging upon the flat beam splitting surface 57a are not constant, but are different from each other. Therefore, it is impossible to accurately split the incident light beam into the P- and S-polarized components. This causes a drawback in that the above mentioned signals obtained from an output of the photodetector 58 might be deteriorated.